The present invention relates to a document scanning device for producing electrical scan signals representative of an image on a document at moving scan points which are moved along corresponding scan lines across the document image. More particularly, the present invention relates to such a scanner in which the document is mounted on a document supporting backing surface and transported past a scanning station defined by the scan points, with the scan signals produced by scanning the document being distinguishable from the scan signals produced by scanning the backing surface. To produce such distinguishable scan signals, the document backing surface bears a characteristic image pattern which produces a recognizable scan signal pattern when the backing surface is scanned.
A document image is scanned to produce electrical scan signals representative of the document image in various types of devices, such as facsimile systems and document duplicator systems. In a number of prior art duplicator systems, the document to be copied is manually placed on a document supporting platen in registration with positional indicia. The document is then optically scanned on the assumption that it is properly positioned. Other prior art duplicator systems, which automatically feed an original document to a scanner, utilize relatively intricate mechanical document handling arrangements to produce proper registration between the document image and the scanner system. Typically, scanning is performed on the assumption that the mechanical registration apparatus has positioned the document appropriately. If a document is mispositioned, however, this may go undetected and the duplicated copy will bear an image which is laterally or longitudinally shifted or skewed with respect to the sheet of copy paper.
While a number of prior art devices have utilized photoelectric detectors to detect the presence or position of a document at a scanning station, or the presence or position of a sheet of paper at a printing station, such devices have detected only the difference between light reflected from a document supporting or backing surface and the document itself. Where documents of varying color and reflectivity are to be scanned, however, it will be appreciated that some documents may have an image adjacent an edge of the document which is substantially the same color or reflectivity as the document backing surface. When this occurs, scanning across the document backing surface and onto the document surface will not result in a substantial change in the amount of light received by the photoelectric detectors and, as a result, the document edge will not be detected.
U.S. Pat. No. 4,060,907, issued Dec. 6, 1977, to Van Hook discloses a printer including a circuit for providing an indication of the location of a sheet of copy paper, mounted on a rotating paper-supporting drum, in which a pair of photoelectric detectors provide signals coincident with the circumferential and axial edges of the sheet of paper. These signals are used to control counters which generate a preset or offset signal value and a gate signal to accommodate axial and circumferential misalignment of the sheet on the drum.
The drum surface is apparently uniformly reflective, with the assumption being made that the sheet of copy paper has a lower reflectivity. Since the sheet of copy paper, prior to printing, bears no print image, the reflectivity of the paper is known and, therefore, the reflectivity of the drum surface can be selected such that there is a substantial difference in the amount of light reflected to the photoelectric detectors from the drum surface and from the surface of the sheet.
Similarly, U.S. Pat. No. 3,684,890, issued Aug. 15, 1972, to Hayne et al, discloses a misfeed detector for sensing the presence of a properly fed sheet prior to the sheet being conveyed to the photoconductive surface of a xerographic machine for transfer of a developed image thereto. A photocell system includes photosensors which detect the reflection of light from an area on a sheet supporting drum where a properly oriented sheet would be positioned. If a misfeed of the sheet is detected, the xerographic machine is shut down. As in the case of the Van Hook device, the misfeed detector of Hayne et al is operable successfully only where a sheet of known reflectivity is scanned.
U.S. Pat. No. 4,171,744, issued Oct. 23, 1979, to Hubbard, discloses a device which photoelectrically scans mixed mail pieces to classify the mail pieces into size and orientation categories for subsequent sorting operations. The apparatus includes a laser beam which scans a conveying belt upon which the documents are transported. Hubbard suggests using a dark nonreflective belt for transporting the letters past the scanning station. It will be appreciated, however, that if a mail piece were to be of a dark color, or a mail piece were to have a portion of its surface printed with dark ink, the scanning system would not provide an accurate indication of the size or orientation of the mail piece.
U.S. Pat. No. 3,439,176, issued Apr. 15, 1969, to Astley et al, discloses a web registration system in which a series of identical images are printed along a web with a black mark being printed in the white space between each of the images. Scanners detect the movement of a white space therepast, followed by the black mark, followed by a second white space, and this detection is utilized to control subsequent printing, folding or cutting of the web. U.S. Pat. No. 2,220,736, issued Nov. 5, 1940, to Stockbarger et al, discloses a photo-optical detector in which a series of transverse marks on a moving web are detected by a pair of photo-optical transducers. The Stockbarger et al device requires that the web have a periodically repeating pattern printed thereon. The web illumination is strobiscopic in nature and is synchronized to the speed of movement of the web.
It is seen, therefore, that a need exists for a scanning system capable of detecting the position of a document being scanned with high reliability and without regard to the document image or the reflectivity of the document.